On any given day, an individual is confronted with a dizzying array of external stimuli. The stimuli can be any combination of visual, aural, tactile, and other types of stimuli, and, alone or in combination, can invoke strong emotions in a given individual. An individual's reactions to received stimuli are important components that define the essence of the individual. Further, the individual's responses to the stimuli can have a profound impact on the mental states experienced by the individual. The mental states of an individual can vary widely, ranging from happiness to sadness, from contentedness to worry, and from calm to excitement, to name a few possible states. Some familiar examples of mental states or emotional states that are often experienced as a result of common stimuli include frustration or disgust during a traffic jam, disappointment from arriving at a shop just after closing time, boredom while standing in line, distractedness while listening to a crying child, delight while viewing a cute puppy video, and impatience while waiting for a cup of coffee. People's mental states influence how they interact with others. Individuals may become perceptive of and empathetic towards those around them based on evaluating and understanding the mental states of the same people. While an empathetic person can easily perceive another person's anxiety or joy and respond accordingly, automated understanding or quantifying of mental states is a far more challenging undertaking. The ability and techniques by which one person perceives another's mental state or states can be quite difficult to summarize or relate to others. In fact when asked to recount how this perception occurs, people often respond by claiming the perceptive feelings originate from a visceral response or “gut feel.”
Many mental states of an individual or group of individuals can be identified and quantified to aid in the understanding of the behavior of the individual or the group of individuals. As one example, individuals who are able to understand their emotional state can choose to use the known mental state information in a variety of practical ways. Similarly, other individuals and observers can use known mental state information about themselves or those around them for the individuals' or observers' own benefit or for the benefits of others. A familiar example is seen in the example of people collectively responding with fear or anxiety after witnessing a catastrophe. Likewise, people can collectively respond with happy enthusiasm when a sports team that they support wins a major victory.